add support for long-opts (#332)

2022-08-16T08:43:50Z

Uses [optparse] to add support for long-opts. optparse is posix compliant with getopt(3) and thus would be backwards compatible. It does not have any dependency (not even the c standard library!) and is C89 compatible and thus fits our current code-style. [optparse]: https://github.com/skeeto/optparse Note that we're using a couple `pragma`-s to silence some harmless warnings. This should be portable because these pragma-s don't change the behavior of the program. Furthermore, C standard mandates that unknown pragma's should be ignored by the compiler and thus would not result in build failure on compilers which do not recognize them. Closes: https://codeberg.org/nsxiv/nsxiv/issues/328 Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/332 Reviewed-by: eylles

nsxiv/optparse.h, branch master

add support for long-opts (#332)